Production of regenerated cellulose films and sheets



Jan. 11, 1955 v F. H. REICHEL ETAL PRODUCTION OF REGENERATED CELLULOSEFILMS AND SHEETS Filed Jan. 19, 1951 ill I uuuuuuuunuunun|||||||| vINVENTORS. FRANK H. RE/CHEL ARTHUR O. RU-S-SELL By M i ATTORNEY.

United States Patent PRODUCTION OF REGENERATED CELLULOSE FILMS ANDSHEETS Frank H. Reichel, Villanova, Pa., and Arthur 0. Russell,Fredericltsburg, Va., assignors to American Viscose Corporation,Wilmington, Del., a corporation of Delaware Application January 19,1951, Serial No. 206,824

Claims. (Cl. 18-57) This invention relates to the production ofregenerated cellulose films, sheets and pellicles and is particularlyconcerned with the production of pellicles of this type having improvedcharacteristics, especially those of strength, swelling, and shrinkage,both transversely and longitudinally, as well as substantial uniformityin thickness.

In the conventional system for the production of regenerated cellulosepellicles from viscose or other cellulosic solution, such ascuprammonium cellulose, the filmforming solution is extruded into acoagulating and/or regenerating bath through a narrow slot, the width ofwhich in commercial practice is tapered down to an even narrower Widthat each end in order to substantially prevent the formation of thickenedlongitudinal edges on the pellicle, commonly referred to as a bead. Thefilm is drawn from the extrusion slot through a series of treatingliquids, generally by the use of suitably disposed guiding rolls, atleast some of which are positively driven to draw the pellicle throughthe system. Some of the rollers are disposed above the baths fortransferring the film from one to another of the baths and some aredisposed in the baths to provide a zig-zag path of travel for the filmthrough the baths thereby enabling the film to pass a considerabledistance through such baths While economizing on space. The nature ofthe baths depends upon the particular type of film-forming solutionused. For example, when it is of viscose, the pellicle may pass from thecoagulating and/ or regenerating bath into a regenerating bath tosubstantially complete the regeneration, then to suitable washing,desnlfurizing, bleaching, neutralizing, and softening baths which followone another in sequence in conventional manner. Because of the forceexerted upon the film by the driving rollers, the film is tensioned inthe machine direction, that is longitudinally, and depending upon therate of speed through the baths, the number of rollers, the length oftravel through the baths, and the particular disposition of the rollerstherein, more or less stretching may be effected upon the film. Theamount of stretch thus inherently imparted to the film in a longitudinaldirection may vary from as low as 5% to as high as 50% or moreelongation relative to the initial length of the film. After passingthrough the liquid treatment baths, the film proceeds in a sinuous pathabout a series of drying rolls which may be heated internally and/or maybe disposed in a closed chamber through which a heated fluid medium iscirculated. At least some of these rolls are driven and the tensionproduced longitudinally in the film prevents any shrinkage thereoflongitudinally. However, some transverse shrinkage occurs and it hasbeen found that the extent of transverse shrinkage varies across thewidth of the film in proportion to the distance from the center thereof,the outer marginal portions shrinking the most. One serious disadvantageof the use of a slot with tapered ends is the fact that the edges areweakened and tear easily. When a tear starts from the edge, itprogresses almost instantly across the entire pellicle interrupting theoperation of the machine.

The product obtained at the completion of the drying step is found tohave distinctly dilferent properties transversely and longitudinally.For example, the marginal areas, especially the outer 3- to 5-inchwidths, do not lie perfectly flat relative to the central area. Instead,they have a slight wavy disposition caused by the fact that the centralsection of the film when measured longitudinally or in the machinedirection is shorter than the marginal areas. This is apparentlyattributable to the fact that the thinner sections near the ends of thematerial extruded 2,698,967 Patented Jan. 11, 1955 set up more quicklythan the thicker central section in the coagulating and/or regeneratingbath and do not have the opportunity to shrink therein to the same eXtent as the thicker and more slowly set-up central section. Also, thetapering width of each end portion of the extrusion slot produces edgeswhich are\not straight and parallel but have an irregular wavy form. Ithas also been found that frequent longitudinal ridges or striations areformed in the marginal areas of the pellicle applarently the result ofthe tapered ends of the extrusion s ot.

Furthermore, when the dry film is wet out with water, an irregularswelling occurs in which the transverse dimension may increase as muchas 30% whereas the longitudinal dimension may shrink as much as 10%. Themarginal portions also have lower tensile strengths and less resistanceto rupture or tearing than the central area.

Numerous proposals have been made previously to impart greateruniformity to the regenerated cellulose pellicles, especially to reducethe anisotropic characteristics pointed out hereinabove. Among them arethose which suggest transversely stretching the pellicle while it isstill in the wet gel condition prior to any drying thereof. Theseprocedures have the disadvantages that the stretching is effected uponthe film while it is in a rather weak and delicate condition and unlessspecial care and equipment is used for effecting such transversestretching, it cannot be satisfactorily and reliably effected upon acontinuously travelling film. Furthermore, during the drying, theirregular shrinkage from the center to the edges of the film stilloccurs and longitudinal tension is unavoidable when dried on a series ofheated rollers.

Another process involved transversely tensioning the film throughout thedrying between edge-gripping belts. This has the disadvantage ofrequiring a special drying equipment to provide for properly controlledtracking of the belts therethrough. Otherwise, the lateral shrinkageforces tend to displace the belts axially along the rollers toward thecenter of the film. Also, up to 6 inches of each edge of the film may bedamaged and require removal.

The principal object of the present invention is to provide an improvedprocedure for reducing the anisotropic properties of the regeneratedcellulose film in a novel and improved manner. A further object is toprovide a procedure for producing regenerated cellulose pellicles whichis accompanied with no sacrifice in strength that is normally derivedfrom the longitudinal tensioning of the film as it proceeds through themachine. A further object is to provide an improved process forproducing regenerated cellulose films having improved characteristicswhich requires relatively simple changes in the equipment conventionallyemployed for the production of such films. Other objects and advantagesof the invention will be apparent from the drawing and the descriptionthereof.

The improved procedure of the present invention is based upon the use ofan extrusion slot having a uniform width throughout its length and uponthe discovery that at a certain narrowly limited range of moisturecontent of the film produced from such a slot in an otherwiseconventional system as described hereinabove, there apparently occurs acritical balance between the combined tensile strength and thicknessfactors of the central portion of the pellicle and the tensile strengthand thickness factors of the marginal areas. This critical balance,which occurs when the film contains from 12 to 15% moisture (based onthe total weight of the humidified cellulose film), is such that themarginal areas of the sheet have a lower resistance to elongation thanthe central areas so that the major effect of transverse tension at thisstage causes an extension of the several unit areas extending widthwiseof the sheet which is generally proportional to the distance of the unitareas from the center of the sheet. At this critical stage of moisturecontent, it has been found that the difierential transverse shrinkageproduced in the previous drying can be partially or entirelycompensated, as desired, by the transverse stretch and the transverseelongation thereby effected substantially remains in the film during thesubsequent drying to the moisture content desired in the final product.

The drawing illustrates a system for executing the inn vention.

In Figure 1, there is shown a diagrammatic side elevation of the system,

Figure 2 shows a plan view of the transverse stretching stagediagrammatically, the extent of stretching being somewhat exaggerated,and

Figure 3 is a detail view showing a form of tenter clip that may beused.

Figure 1 shows a hopper 3 which feeds the film-forming solution, such asviscose, through a slot orifice 4 in the extrusion device disposed atthe bottom of the hopper. It is characteristic of the present inventionthat the width of the slot is constant throughout its length to producethe necessary thickness throughout the width of the film to adapt thefilm to the subsequent transverse stretching that constitutes another ofthe vitally important elements of the present invention. Ancillaryadvantages are also derived from the uniform slot including a greatlyreduced frequency of the occurrence of tears and breaks which interruptoperation, and also the substantial elimination of the longitudinalridges and striations. The film 5 proceeds downwardly through thecoagulating and/ or regenerating bath in the container 6 and thenthrough a series of treating baths such as that shown contained in thevessel 7. Passage through the various liquid-treating baths isfacilitated by means of rollers S at least some of which, preferablythose above the baths, being positively driven. If desired, stationaryrods may replace the rollers disposed in the baths. Generally, therollers 8 are all driven at substantially the same speed unless it isdesired to deliberately introduce longitudinal stretch in addition tothat which is inherently produced by the drawing forces exerted by thedriven rollers.

From the wet treating stages, the film passes about the drying cans ordrums 9, some or all of which are positively driven. The speed of travelof the film about the drying cans and the drying conditions associatedtherewith are controlled to reduce the moisture of the film to a valueof 4 to based on the weight of the cellulose. Generally, it is preferredto dry down to 4 to 8% moisture content, such as that which isordinarily contained in regenerated cellulose pellicles when exposed toordinary atmospheric conditions.

When the film is dried to a moisture content of 12 to 15% at this stage,it may pass about a guide roll 16 directly into a transverse stretchingstage at 11 which may be contained within the housing 12 having theentrance 13 and the discharge port 14 for the film. Any suitable devicefor stretching the film transversely may be disposed within the chamber12. As diagrammatically shown, this device may comprise a clip tenterframe having the edge-gripping clips carried on a pair of oppositelydisposed chains 15 and 16. Since tenter frames are conventional, it isnot considered necessary to describe the specific details thereof.Essentially, they consist of a series of clips provided on travellingchains or belts which operate above complementary blocks which serve asgripping tracks, that is they are adapted to cooperate with the clipsthereabove to grip the edge of the film or sheet therebetween. Figure 3illustrates diagrammatically, as viewed in the general longitudinaldirection of the film, the edgegripping arrangement. It comprises ablock 17 which may have a groove or depression 18 against which the edge19 of the film is urged by the jaw 20 of the clip which is pivotallymounted on the axis 21 and urged into gripping engagement by the spring22 acting against the stationary member 23 and the arm 24 extending backfrom the clip. The head 5a at the edge of the film 5 assists in thegripping action and gives the necessary protection against tears andbreaks, thereby rendering such occurrences infrequent.

Preferably, in the embodiment in which the film contains 12 to 15moisture content when discharged from the drums 9, it is passed throughan entrance slit 140 and through an ante chamber 12a, in which it maytravel about rolls 8a, before entering the stretching stage in chamber12. In chamber 12a, moisture equalization is allowed to occur, that isduring passage through this chamber, any inequalities in the moisturedistribution in the film are reduced or disappear entirely.

Although it is not absolutely necessary, it is preferred to circulatethe moist atmosphere which develops adjacent the film in the chamber12a. This may be effected by means of the circulating fan 25 connectedon its suction side by the inlet duct 26 to the top of the chamber andhaving its discharge side connected by the conduit 27 to the bottom ofthe chamber 12 preferably at the opposite end of the chamber to thatwhere conduit 26 is connected. If desired, instead of merelyrecirculating the moist air developed by the film in the chamber 12a,such air may be reheated as by means of a coil 28 disposed in eitherconduit 26 or 27 and if desired further, the reheated air may passthrough a humidifying device 29 which may comprise water sprays or thelike for the purpose of maintaining the proper humidity in chamber 12a.

From the transverse stretching stage 11, the stretched film passes aboutthe final drying cans 3i) and then after attaining a moisture content ofapproximately 6% or less based on the weight of the cellulose therein,the film proceeds to a take-up, such as a winding device at 31. Ifdesired, the film, on its way to the take-up 31, may pass through theedge-slitting device comprising rotary knives 32 which coact with acooperating roll 33.

In the preferred system of operation wherein the film discharged fromthe drying drums 9 has a moisture content below 12% by weight, such as 4to 8% by weight, the chamber 12a in cooperation with the humidifier 29serves to re-humidify the film to the desired moisture content of 12% upto 15% before it enters the stretching stage in chamber 12.

The clip chains of the tenter diverge from the point where the filmenters and is first gripped to the discharge point where the film isreleased from the clips. The amount of transverse stretching effectedmay be varied from a minimum of about 4% wherein the sheet is freed ofany wrinkling caused by previous transverse shrinkage to a value whichsubstantially completely compensates or may even overcompensate for theprevious transverse shrinkage. This value may vary depending upon theparticular thickness and width of the pellicle and its history ofmanufacture. In a typical embodiment, the film is discharged from dryingdrums with a normal moisture content of 4 to 8% by Weight, at whichstage it has been stretched longitudinally but not transversely. and hasbeen allowed no shrinkage longitudinally but has shrunk approximately10% in width. In chamber 12a, it is humidified to a moisture content of12% by Weight in an atmosphere having a temperature of 165 F. and ahumidity of about 99% of saturation. In this chamber no longitudinalshrinkage occurs but the film expands transversely about 2 to 3%,thereby partially compensating for the earlier transverse shrinkageduring drying. In the chamber 12, the moisture content remains constantat 12% and while neither stretch nor shrinkage occurs longitudinally, atransverse stretch of about 7% is applied, thereby substantiallycompensating completely for the remainder of the shrinkage during theoriginal drying. During the subsequent drying on drums 30, there issubstantially no stretch or shrinkage either longitudinally ortransversely. If a transverse shrinkage in the first drying stageamounts to more than 10%, such as 15%, the transverse stretch applied ispreferably increased to compensate entirely for this amount of shrinkageafter taking into consideration the amount of transverse expansionoccurring in chamber 12a.

The final product is characterized by improved strength transverselywithout any appreciable loss in strength lonitudinally. Also, itsswelling and stretch characteristics are more nearly alike transverselyand longitudinally. The uniformity of gauge is greatly improved, thesheet being substantially uniform in thickness throughout its widthexcept at the very edges where the customary beads occur and may beremoved by conventional slitters as mentioned above. Also, the marginalareas lie substantially flat in the same plane as the central area ofthe sheet and are free of longitudinal ridges or striations in themarginal area. The pellicle exhibits substantially the same resistanceto tearing in the marginal areas as in the central portion of the sheet.

it is to be understood that changes and variations may be made withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

We claim:

1. A process for producing regenerated cellulose films comprisingextruding viscose through a slot having uniform width throughout itslength into a liquid coagulating bath, drawing the extruded film througha plurality of liquid baths comprising a regenerating bath, and afterdrying the liquid-treated regenerated cellulose film to a moisturecontent of 4 to 8% by weight while preventing longitudinal shrinkage andallowing transverse shrinkage varying in magnitude depending upon thedistance from the center line of the film, humidifying the film to amoisture content of 12 to 15% by weight, transversely stretching thehumidified film between 4 and 7% While preventing longitudinal shrinkageand stretching, and subsequently drying the transversely stretched filmto a normal moisture content.

2. A process as defined in claim 1 in which the film is humidified to12% moisture content and transverse stretching is efiected upon the filmwhen it contains approximately 12% by weight moisture.

3. A process as defined in claim 2 in which the film undergoessubstantially no transverse shrinkage or stretching and substantially nolongitudinal shrinkage or stretching during the final stage of drying.

4. A process for producing regenerated cellulose films comprisingextruding viscose through a slot having uniform width throughout itslength into a liquid coagulating bath, drawing the extruded film througha plurality of liquid baths comprising a regenerating bath, and afterdrying the liquid-treated film to a moisture content of 4 to 8% byweight while preventing longitudinal shrinkage and allowing transverseshrinkage varying in magnitude depending upon the distance from thecenter line of the film, humidifying the film to a moisture content of12 to 15 by weight, effecting moisture equalization in the presence of acirculating humid atmosphere, and then transversely stretching thehumidified film between 4 and 7% while preventing longitudinal shrinkageand stretching, and subsequently drying the transversely stretched filmto a normal moisture content.

5. A process as defined in claim 4 in which the moisture distributed bythe equalization is derived substantially entirely from the filmentering the equalization zone.

References Cited in the file of this patent UNITED STATES PATENTS1,975,708 Bleibler Oct. 2, 1934 1,979,762 OKane et al Nov. 6, 19342,115,132 Alles et a1 Apr. 26, 1938 2,144,356 Alles Jan. 17, 1939

1. A PROCESS FOR PRODUCING REGENERATED CELLULOSE FILMS COMPRISINGEXTRUDING VISCOSE THROUGH A SLOT HAVING UNIFORM WIDTH THROUGHOUT ITSLENGTH INTO A LIQUID COAGULATING BATH, DRAWING THE EXTRUDED FILM THROUGHA PLURALITY OF LIQUID BATHS COMPRISING A REGENERATING BATH, AND AFTERDRYING THE LIQUID-TREATED REGENERATED CELLULOSE FILM TO A MOISTURECONTENT OF 4 TO 8% BY WEIGHT WHILE PREVENTING LONGITUDINAL SHRINKAGE ANDALLOWING TRANSVERSE SHRINKAGE